In heart, rapid and transient changes in intracellular [Ca2+] are fundamental to cell function. Rapid Ca2+ release from the sarcoplasmic reticulum through ryanodine receptor (RyR) channels controls contractility. The RyR channel, however, is not the only intracellular Ca2+ release channel present in isolated heart cells. Like all mammalian cells, ventricular cardiac myocytes contain a significant number of IP3Rs. The IP3Rs are intracellular Ca2+ release channels that are homologous to the RyR channel. The RyR and IP3R have similar permeation properties but are differentially regulated by Ca2+ and IP3. The RyR and IP3R mediated intracellular Ca2+ signaling pathways must operate simultaneously in the same cell. How these pathways can operate simultaneous and maintain their individual signal fidelity is not yet clear. In this proposal, single channel function of cardiac RyR and IP3R Ca2+ release channels will be defined in planar bilayers. The kinetics of single RyR and IP3R channels as they respond to fast, calibrated Ca2+ and IP3 stimuli will measured. Fast ligand stimuli will be generated by flash photolysis. The possibility that phosphorylation impacts the differential regulation of the two types of channels will also be investigated. The overall goal is to establish key determinants of cardiac RyR and IP3R function at the single channel level. This represents an important step (i.e. defining/contrasting RyR and IP3R regulatory mechanisms) in understanding the complexity of local intracellular Ca2+ signaling in the heart. Aim number1: Determine the location, identify and single channel function of IP3R channels in isolated ventricular myocytes. Aim number 2: Define the identity and kinetics of the mechanisms that "turn-on" and "turn-off" single RyR and IP3R release channels. Aim number 3: Determine how PKA and PKC dependent phosphorylation impacts function of single RyR and IP3R channels.